We are investigating approaches for utilizing recombinant DNA techniques for developing vaccines to human respiratory syncytial virus (RSV). Previously, recombinant vaccinia viruses or adenoviruses were constructed that express individual RSV proteins including the major protective antigens, the F and G glycoproteins. Efficacy tests in rodents and monkeys provided highly encouraging results, but in chimpanzees the recombinants proved to be insufficiently immunogenic and protective. Improvements in vector technology will be required before this approach will be feasible for RSV immunoprophylaxis. Construction of a series of adenovirus vectors containing alternative engineered insertion sites did not yield improved levels of expression of immunogenicity in rodents. A second approach involves efforts to develop methods for producing infectious RSV from cDNA. This would make it possible to produce and characterize attenuated strains by direct genetic engineering. To date, we have successfully "rescued" a cDNA-encoded vRNA analog which is 49.3% of full-length and which contains an enzyme marker gene under the control of RSV transcriptive signals. Standard infectious RSV was used as helper to provide proteins to complement the cDNA-encoded vRNA analog. This analog was packaged into infectious particles and was passed five times in culture with undiminished efficiency. This provides support for the idea that a complete, nondefective cDNA-encoded vRNA can be rendered biologically- active to yield infectious virus.